Should services always return DTOs, or can they also return domain models?
I'm (re)designing large-scale application, we use multi-layer architecture based on DDD.
We have MVC with data layer (implementation of repositories), domain layer (definition of domain model and interfaces - repositories, services, unit of work), service layer (implementation of services). So far, we use domain models (mostly entities) across all layers, and we use DTOs only as view models (in controller, service returns domain model(s) and controller creates view model, which is passed to the view).
I'v read countless articles about using, not using, mapping and passing DTOs. I understand that there's no any definitive answer, but I'm not sure if it's ok or not returning domain models from services to controllers. If I return domain model, it's still never passed to the view, since controller always creates view-specific view model - in this case, it seem legit. On the other hand, it doesn't feel right when domain model leaves business layer (service layer). Sometimes service needs to return data object that wasn't defined in the domain and then we either have to add new object to the domain that isn't mapped, or create POCO object (this is ugly, since some services return domain models, some effectively return DTOs).
The question is - if we strictly use view models, is it ok to return domain models all the way to controllers, or should we always use DTOs for communication with service layer? If so, is it ok to adjust domain models based on what services need? (Frankly I don't think so, since services should consume what domain has.) If we should strictly stick to DTOs, should they be defined in service layer? (I think so.) Sometimes it's clear that we should use DTOs (e.g., when service performs lot of business logic and creates new objects), sometimes it's clear that we should use just domain models (e.g., when Membership service returns anemic User(s) - it seems it wouldn't make much sense to create DTO that is the same as domain model) - but I prefer consistency and good practices.
Article Domain vs DTO vs ViewModel - How and When to use them? (and also some other articles) is very similar to my problem, but it doesn't answer this question(s). Article Should I implement DTOs in repository pattern with EF? is also similar, but it doesn't deal with DDD.
Disclaimer: I don't intend to use any design pattern only because it exists and is fancy, on the other hand, I'd like to use good design patterns and practices also because it helps designing the application as a whole, helps with separation of concerns, even though using particular pattern isn't "necessary", at least at the moment.
it doesn't feel right when domain model leaves business layer (service layer)
Makes you feel like you are pulling the guts out right? According to Martin Fowler: the Service Layer defines the application's boundery, it encapsulates the domain. In other words it protects the domain.
Sometimes service needs to return data object that wasn't defined in the domain
Can you provide an example of this data object?
If we should strictly stick to DTOs, should they be defined in service layer?
Yes, because the response is part of your service layer. If it is defined "somewhere else" then the service layer needs to reference that "somewhere else", adding a new layer to your lasagna.
is it ok to return domain models all the way to controllers, or should we always use DTOs for communication with service layer?
A DTO is a response/request object, it makes sense if you use it for communication. If you use domain models in your presentation layer (MVC-Controllers/View, WebForms, ConsoleApp), then the presentation layer is tightly coupled to your domain, any changes in the domain requires you to change your controllers.
it seems it wouldn't make much sense to create DTO that is the same as domain model)
This is one of the disadvantage of DTO to new eyes. Right now, you are thinking duplication of code, but as your project expands then it would make much more sense, specially in a team environment where different teams are assigned to different layers.
DTO might add additional complexity to your application, but so are your layers. DTO is an expensive feature of your system, they don't come free.
Why use a DTO
This article provides both advantage and disadvantage of using a DTO, http://guntherpopp.blogspot.com/2010/09/to-dto-or-not-to-dto.html
Summary as follows:
When to Use
For large projects.
Project lifetime is 10 years and above.
Strategic, mission critical application.
Large teams (more than 5)
Developers are distributed geographically.
The domain and presentation are different.
Reduce overhead data exchanges (the original purpose of DTO)
When not to Use
Small to mid size project (5 members max)
Project lifetime is 2 years or so.
No separate team for GUI, backend, etc.
Arguments Against DTO
Duplication of code.
Cost of development time, debugging. (use DTO generation tools http://entitiestodtos.codeplex.com/)
You must synchronize both models all the time. (personally, I like this because it helps know the ripple effect of the change)
Cost of developement: Additional mapping is necessary. (use auto mappers like https://github.com/AutoMapper/AutoMapper)
Why are data transfer objects (DTOs) an anti-pattern?
Arguments With DTO
Without DTO, the presentation and the domain is tightly coupled. (This is ok for small projects.)
Interface/API stability
May provide optimization for the presentation layer by returning a DTO containing only those attributes that are absolutely required. Using linq-projection, you don't have to pull an entire entity.
To reduce development cost, use code-generating tools
I'm late to this party, but this is such a common, and important, question that I felt compelled to respond.
By "services" do you mean the "Application Layer" described by Evan's in the blue book? I'll assume you do, in which case the answer is that they should not return DTOs. I suggest reading chapter 4 in the blue book, titled "Isolating the Domain".
In that chapter, Evans says the following about the layers:
Partition a complex program into layers. Develop a design within each layer that is cohesive and that depends only on the layers below.
There is good reason for this. If you use the concept of partial order as a measure of software complexity then having a layer depend on a layer above it increases complexity, which decreases maintainability.
Applying this to your question, DTOs are really an adapter that is a concern of the User Interface / Presentation layer. Remember that remote/cross-process communication is exactly the purpose of a DTO (it's worth noting that in that post Fowler also argues against DTOs being part of a service layer, although he isn't necessarily talking DDD language).
If your application layer depends on those DTOs, it is depending on a layer above itself and your complexity increases. I can guarantee that this will increase the difficulty of maintaining your software.
For example, what if your system interfaces with several other systems or client types, each requiring their own DTO? How do you know which DTO a method of your application service should return? How would you even solve that problem if your language of choice doesn't allow overloading a method (service method, in this case) based on return type? And even if you figure out a way, why violate your Application Layer to support a Presentation layer concern?
In practical terms, this is a step down a road that will end in a spaghetti architecture. I've seen this kind of devolution and its results in my own experience.
Where I currently work, services in our Application Layer return domain objects. We don't consider this a problem since the Interface (i.e. UI/Presentation) layer is depending on the Domain layer, which is below it. Also, this dependency is minimized to a "reference only" type of dependency because:
a) the Interface Layer is only able to access these Domain objects as read-only return values obtained by calls to the Application layer
b) methods on services in the Application Layer accept as input only "raw" input (data values) or object parameters (to reduce parameter count where necessary) defined in that layer. Specifically, application services never accept Domain objects as input.
The Interface Layer uses mapping techniques defined within the Interface Layer itself to map from Domain objects to DTOs. Again, this keeps DTOs focused on being adapters that are controlled by the Interface Layer.
CustomerApplicationService.Save(<input>) if you dont know which properties where changed (the customer can have tens of properties) ?
In my experience you should do what's practical. "The best design is the simplest design that works" - Einstein. With that is mind...
if we strictly use view models, is it ok to return domain models all the way to controllers, or should we always use DTOs for communication with service layer?
Absolutely it's ok! If you have Domain Entities, DTO's and View Models then including database tables you have all the fields in the application repeated in 4 places. I've worked on large projects where Domain Entities and View Models worked just fine. The only expception to this is if the application is distributed and the service layer resides on another server in which case DTOs are required to send across the wire for serialization reasons.
If so, is it ok to adjust domain models based on what services need? (Frankly I don't think so, since services should consume what domain has.)
Generally I'd agree and say no because the Domain model is typically a reflection of the business logic and doesn't usually get shaped by the consumer of that logic.
If we should strictly stick to DTOs, should they be defined in service layer? (I think so.)
If you decide to use them I'd agree and say yes the Service layer is the perfect place as it's returning the DTOs at the end of the day.
Good luck!
It seems that your application is big and complex enough as you have decided to go through DDD approach. Don't return your poco entities or so called domain entities and value objects in you service layer. If you want to do this then delete your service layer because you don't need it anymore! View Model or Data transfer objects should live in Service layer because they should map to domain model members and vice versa. So why do you need to have DTO? In complex application with lots of scenarios you should separate the concerns of domain and you presentation views, a domain model could be divided into several DTO and also several Domain models could be collapsed into a DTO. So it's better to create your DTO in layered architecture even it would be the same as your model.
Should we always use DTOs for communication with service layer? Yes, you have to return DTO by your service layer as you have talk to your repository in service layer with domain model members and map them to DTO and return to the MVC controller and vice versa.
Is it ok to adjust domain models based on what services need? A service just talks to repository and domain methods and domain services, you should solve the business in your domain based on your needs and it's not the service task to tell the domain what is needed.
If we should strictly stick to DTOs, should they be defined in service layer? Yes try to have DTO or ViewModel just in service later because they should be mapped to domain members in service layer and it's not a good idea to places DTO in controllers of your application(try to use Request Response pattern in your Service layer), cheers!
So far, we use domain models (mostly entities) across all layers, and we use DTOs only as view models (in controller, service returns domain model(s) and controller creates view model, which is passed to the view).
Since Domain Model provides terminology (Ubiquitous Language) for whole your application it is better to use Domain Model widely.
The only reason to use ViewModels/DTOs is an implementation of MVC pattern in your application to separate View (any kind of presentation layer) and Model (Domain Model). In this case your presentation and domain model are loosely coupled.
Sometimes service needs to return data object that wasn't defined in the domain and then we either have to add new object to the domain that isn't mapped, or create POCO object (this is ugly, since some services return domain models, some effectively return DTOs).
I assume that you talk about Application/Business/Domain Logic services.
I suggest you return domain entities when you can. If it is needed to return additional information it is acceptable to return DTO that holds several domain entities.
Sometimes, people who use 3rd part frameworks, that generates proxies over domain entities, face difficulties exposing domain entities from their services but it is only a matter of wrong usage.
The question is - if we strictly use view models, is it ok to return domain models all the way to controllers, or should we always use DTOs for communication with service layer?
I would say it is enough to return domain entities in 99,9% cases.
In order to simplify creation of DTOs and mapping your domain entities into them you can use AutoMapper.
Late to the party, but I’m facing the exact same type of architecture and I’m leaning towards “only DTOs from service”. This is mainly because I’ve decided to only use domain objects/aggregates to maintain validity within the object, thus only when updating, creating or deleting. When we’re querying for data, we only use EF as a repository and maps the result to DTOs. This makes us free to optimize read queries and not adapt them to business objects, often using database functions as they are fast.
Each service method defines its own contract and is therefore easier to maintain over time. I hope.
If you return part of your domain model, it becomes part of a contract. A contract is hard to change, as things outside of your context depend on it. As such, you would be making part of your domain model hard to change.
A very important aspect of a domain model is that it is easy to change. This makes us flexible to the domain's changing requirements.
I'd suggest analyzing these two questions:
Are your upper layers (i.e. view & view models / controllers) consuming the data in a different way of what the domain layer exposes? If there is a lot of mapping being done or even logic involved I'll suggest revisiting your design: it should probably be closer to how the data is actually used. How likely is it that you deeply change your upper layers? (e.g. swapping ASP.NET for WPF). If this is highly unlike and your architecture is not very complex, you may be better off exposing as many domain entities as you can.
I'm afraid it is quite a broad topic and it really gets down to how complex your system is and its requirements.
In my experience, unless you are using an OO UI pattern (like naked objects), exposing the domain objects to the UI is a bad idea. This because as the application grows, the needs from the UI change and force your objects to accommodate those changes. You end up serving 2 masters: UI and DOMAIN which is a very painful experience. Believe me, you don't want to be there. The UI model has the function of communicating with the user, the DOMAIN model to hold the business rules and the persistence models deals with storing data effectively. They all address different needs of the application. I'm in the middle of writing a blog post about this, will add it when it's done.
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